Unmanned aerial vehicle (UAV) or micro-air vehicle (MAV) applications that rely on a vacuum from the engine to draw the fuel, as opposed to having a pump in the gas reservoir, may generate air bubbles in the fuel line. In addition, UAVs often encounter turbulence and adverse flying conditions that may introduce air into the fuel line such as when the vehicle pitches or engages in forward flight on a low fuel reserve. Similarly, fuel drawn by a vacuum in an operating environment like the desert where there are elevated temperatures is particularly susceptible to vapor lock caused by air in the fuel line. As such, any of these situations may result in the loss of an air vehicle by starving the engine of fuel.
One apparatus employed to regulate the proper amount of fuel fed to the engine is the carburetor. Carburetors typically contain a float chamber that holds a quantity of fuel for immediate use. This reservoir is constantly replenished with fuel supplied by a fuel pump. The correct fuel level in the chamber is maintained by a float that cooperates with the opening of an inlet valve. As the fuel level is depleted by the engine, the float drops accordingly, opening the inlet valve and admitting fuel. As the fuel level rises, the float rises and seals the inlet valve. Usually, ventilation tubes allow air to escape from the chamber as it fills with fuel or air to enter as the chamber empties, maintaining atmospheric pressure within the float chamber.
However, where the engine may be operated in various orientations relative to the ground, such as in a UAV, the float chamber of a carburetor is rendered useless due to its dependence on gravity. In addition, there is a significant weight associated with the fuel pump and float chamber, which is a key reason for not utilizing them in UAVs. To solve this problem, a flexible diaphragm may be utilized to form a wall of the fuel chamber so that as fuel is drawn into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to a needle valve and as the diaphragm moves into the chamber the needle valve is forced open to admit more fuel. As fuel enters the chamber, the diaphragm expands outward, closing the needle valve. A balanced state is reached which creates a steady fuel level that remains constant in any orientation. This modified carburetor lacks an air venting mechanism and thus does not resolve the vapor lock issue presented by UAVs.